Chroma key and a few images for background
You can use Maths nodes to set up logic that allows you to select an image based on an input and this can be used instead of the image sequence or video. For example, consider the following nodes :



The images are set in each of the Image nodes highlighted. To corresponding Greater Than nodes should each have incremental values and these will control the Mix such that subsequent images are selected as the input Value is increased. ie, Value less than or equal to 1 selects the first image, less than or equal to 2 selects the second image, 3 the third image, etc. Continue the chain for as many images as you require.

All that is now required is to keyframe the input Value so that it selects the relevant image for the frame in your animation.